Digestion of fibrous material



` July 11, 1933. H. G. CRAM DIGESTION 0F FIBROUS MATERIAL Filed Dec. 3l, 1932 mmm Iveaz'oa: Hervey 0210222),

Patented July `11, 1933` UNITED STATES EEBVEY G. CHAI, OF BUCSPORT, IAIN E, ASSIGN'OR TO MAINE SEABOARD PAPER COIMI- PATENT oFFlcE PANY, O1' BUCKSPORT, MAINE, A CORPORATION 0F MAINE DIGESTION OF FIBROUS IMIA'JPIELYBIIAIIC` Application. led December 31, 1982. Serial No. 649,685.

This invention relates to the digestion of fibrous material as practiced, .for exam le,

in the reparation of pul for the manu acture o paperfand the object is to provide a process more eilicient and rapid than those hitherto known.

My invention will be well understood from the description of an illustrative embodiment thereof taken in connection -with the accompanying drawing, wherein I have shown diagrammatically a digester installation.

For convenience I shall lirst describe didactically one form of my process as applied to the preparation of paper pulp by the sulphit process, although not limited thereto,

and likewise for convenience will use specic quantitative figures in the description which are to betaken, however, as illustrative merely and not as limitations. g t

`Referring to the drawing, the di ester 3 there shown maybe filled with suita 1y prepared chips. The cover is fastenedsecurely, valve 5 to the overflow line is opened and acid is pumped from storage tank 7 by means of pump 9 into the bottom ofthe digester until it is completely filled, as indicated by the How of acid through the overflow line, at which time the valve 5 is closed. Pumping may now be continued until the pressure capacity of the pump 9 is reached and a high pressure booster pump 11 may then take the burden and be continued in operation until a pressure is reached in the digester preferably markedly in excess of the pressure at which the cooking is to take place, for example, a pressure in the neighborhood of 125 pounds per square inch gage. By this means the acid is forced into the interior of the chips, thorou hly saturating the same, as is indicated by t e fact that approximately 20% additional acid isv added to the digester by means of this pressure, the excess being absorbed in the interior of the chips. Since the digester is completely filled with liquid and the pressure of the pump is exerted through a column of the liquid in the connecting piping, the chips are completely submerged. There 1s no opportunity for any to float on the surface of the liquor resulting in incomplete Apenetration or later causing treatment under a pressure at least substantially equal to the cooking pressure, which for purposes of illustration mi ht be considered as in the neighborhood o 75 pounds, and preferably much greater as described. The cooking operation is preferably carried on with a gas space at the top of the digester and this may conveniently be providedat this point in the process prior to the admission of the cooking steam and without release of the pressure belowy the desired minimum by admitting compressed air at a suitable pressure, say 125 pounds, from air line-13 to the top of the digester, valve 1 5`being opened to permit the discharge of a predeterminedamount of acid 'from the digester back to the storage tank 7. p

The chips being, as it were, water-logged by the'a'cid forced into them under pressure, will sink, say to the level denoted by the line a in the digester, and the acid retained in the digester should be suiicient completely to cover these chips, say to the level indicated by the line b, whereupon valve 15 is closed and the air shut of. Experience has shown that the chips will continue to absorb acid` actuated by diaphragm motor 23. The pres sure within the digester is maintained at a substantially constant value, say 75 pounds gage, by means of a discharge valve 25 in a discharge line leading to gas cooler 27 and thence back to the storage tank 7. I have herein shown this valve as operated by diaphragm motor 29 under automatic control of an instrument 31 of the recorder controller type and operating responsively to the pressure conditions in the digester regardless of the amount of steam supplied.

It will thus be see that the initial pressure placed upon the acitl forcing the same intoV the chips is not released at any time lult is maintained from the beginning and at a value preferably at least as great as the desired cooking pressure and that this pressure is maintained with minimum variation throughout the entire period of cook.

It is important for reasons which will appear that the differential in pressure between the incoming steam and the air space at the top of the digester be as small as possible. This can be accomplished by suitably increasing the size of the piping at the inlet to the digester, as indicated in the figure, so as to obtain the required volume of steam at relatively low pressure and low velocity and therefore the required heat units for cooking. It will be understood that the rate of flow of steam supplied necessarily varies at different stages of the cook and this may be governed manually or automatically by suitable design i of the controlling instrument 19. Instruments which serve to control the operation of a diaphragm motor such as 23 in accordance with a curve whose coordinates are time and some other variable, as, for instance, rate of flow, are well known and do not need extended description herein.

In accordance with the process herein described, the pressure originally applied to the'charge not only resultsv in efficient penetration of the chips, thereby to abbreviate and improve the cooking process, but once this pressure'is applied it is maintained substantially const-ant throughout the duration of the cook. There is no great differential in pressure between the incoming steam line and the air space at the top of the digester. The steam enters gently and instead of tending to bore its way through the charge along the center of the digester resulting in poor heating along the sides, the heat is distributed uniformly throughout the charge. Relatively little disturbance of the liquid results due to the gentle admission of the steam and consequently, referring particularly to the sulphite process, relatively little sulphur dioxide gas is lost from the acid to be discharged to the air space at the top from which it must be relieved weakening the acid and affecting the quality of the resultant pulp. As contrasted with the known processes in which the digester is at substantially atmospheric pressure when steam is admit-ted and in which the admission of steam to bring the digester to cooking temperature must, to avoid boring and violent agitation of the contents of the digester, be gradual and effected over a period of hours with const-ant watchfulness,l the temperature may be easily and promptly l raised to the desired point with resultant abbreviation of the time required for completing the cook. My process thus providesquickly cooking, better circulation of heat units throughout the liquid contents of the digester, conservation of the acid strength Within the digest-er and more thorough cooking of the Wood content by reason of the greater penetration of the chips due to the initially applied pressure. The cooking operation in accordance with the process described is so much simplified, adapting it to be efficiently carried out if desired, under automatic control by instruments of known ty e.

I am aware that the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and I therefore desire the present embodiment to be considered in all respects as illustrative and not restrictive; reference being had to the appended claims rather than to the foregoing description to indicate the scope of the invention.

I claim:

l. In the digestion of fibrous material the method which comprises com letely filling the digester containing the brons charge with liquor under a pressure at least approximating the desired cooking pressure and introducing steam thereto to effect the cook While substantially maintaining such pressure.

2. In the digestion of fibrous material the method which comprises completely filling the digester containing the fibrous chargev vwith liquor under a pressure at least approriimating the desired cooking pressure, dlscharging a portion of the liquor without substantial fall of such pressure and introducing steam thereto to effect the cook while substantially maintaining such pressure.

3. In av digestion of fibrous material the method which comprises completely filling the digester containing' the fibrous charge with liquor under-a pressure at least approximating the desired cooking pressure, discharging a portion of the liquor by admission of gas under pressure to provide a gas space and introducing steam thereto to effect the'cook .Wh/ile substantially maintaining such pressure.

4. In the digestion of fibrous material the method which comprises saturating the chips with liquor` by anelevated pressure applied through a column thereof and thereupon introducing steam to effect the cook while maintaining a pressure substantially constant throughout the cooking period.

5. In the digestion of fibrous material the method which comprises saturating the chips With'liquor by an elevated pressure applied through a column thereof, introducing steam to effect the cook while maintaining a pressure of several atmospheres and regulating the cooking pressure by relieving the same tov maintain a substantially constant Value durinlg the cook. 6. n

the digestion of fibrous material the method which comprises saturating the chips with liquor at a pressure substantially exceeding the cooking pressure and exerted' through a column of theliquor and intro- In testimony whereof, I have signed my name to this specification.

HERVEY e. CRAM. 

